Operational amplifiers used in low-voltage CMOS (complementary metal oxide semiconductor) integrated-circuit processes normally require a differential input stage that can operate properly even when the common-mode voltage is near the supply rails. This is because when the common-mode input voltage is near the supply rails, it can cause changes in internal bias points. This, in turn, can lead to degradation in performance of the input pair and consequently the entire amplifier.
A rail-to-rail differential input stage may be used to deal with the common-mode voltage problem. Typically, a rail-to-rail differential input stage is based on the use of a p-channel and an n-channel complementary differential pair of transistors. If only a PMOS (p-type metal oxide semiconductor) or an NMOS (n-type metal oxide semiconductor) differential input stage is used, then the common mode range does not span both of the rails. If only an NMOS input stage is used then the common mode range (CMR) is limited on the lower side. Conversely, if only a PMOS input stage is used, the CMR is limited on the higher side. Commonly, the technique used to achieve rail-to-rail CMR is to use both an NMOS and a PMOS differential pair in the input stage.